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Fish Diseases and Pest Control

A fish gets sick when a germ (virus, bacterium, fungus or parasite), a weak fish and bad conditions meet. Diseases spread through water, contact and equipment. Farmers diagnose by looking at behaviour and body signs and by lab tests, then control with prevention first (clean water, fewer fish, quarantine, vaccines) and medicine only when needed.

🎬 Step-by-step story

  1. This is a fish pond. All fish are orange and healthy. They swim freely.
  2. A germ enters one fish. It turns red. That fish is now sick.
  3. Fish touch each other and swim through the same water. The germ jumps to more fish. More turn red.
  4. The farmer looks at the fish, checks them, and moves the sick fish into a separate tank. This stops the spread.
  5. Now the farmer keeps the water clean and keeps fewer fish. The sick fish slowly get well.
  6. Your turn. Move the fish slider and the water slider. Crowded fish and dirty water make the germ spread faster.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why do healthy fish not get sick without a germ?

Infectious disease needs a germ. Without one, fish may still get sick from poor water or food, but that is a different kind and does not spread.

How does a germ first get into a pond?

Through infected young fish, wild fish, birds, water from outside or dirty nets. In the 3D one red fish stands for that first sick fish.

Why do more fish turn red over time?

Fish touch and share water, so the germ jumps from one fish to the next. Each new sick fish also spreads it.

Why move sick fish to a separate tank?

It stops them touching healthy fish and lets you look at them closely and treat only them.

How can clean water help sick fish recover?

Good oxygen and low ammonia reduce stress, and a strong fish body can fight the germ better. Fewer fish also means less touching.

Why does crowding make disease spread faster?

More fish in the same space means more touching, dirtier water and more stress. Try the fish slider at 20 and see.

Types of diseases and how they spread

Fish diseases are of two kinds. Infectious diseases are caused by living germs. Non-infectious diseases come from bad food, poison or poor water, and they do not pass from fish to fish.

How they spread: through water, by touching, through wild fish, by nets and boots, by infected young fish or by dirty feed. Crowding, dirty water, stress and wrong temperature make fish weak and the germ strong. This is the disease triangle: germ, host fish and environment.

Pests are animals or plants that harm the farm: parasites, birds and snakes that eat fish, crabs that dig pond walls, and weeds that use up oxygen.

Diagnosis: finding out what is wrong

Good farmers watch every day. Signs of sickness: fish stop eating, swim at the surface or on their side, rub on walls, show red spots, ulcers, torn fins, pale gills or a swollen belly, and many die quickly.

Steps of diagnosis:

  1. Look at behaviour and the pond water.
  2. Test water: oxygen, pH, ammonia, temperature.
  3. Take a few sick fish and look at skin and gills under a microscope.
  4. Lab tests, such as culture of bacteria or PCR for viruses, find the exact germ.

Only when the cause is known is the right action chosen.

Control: prevent first, treat second

Prevention (cheapest and best):

Treatment: remove and move sick fish, then use medicine only if a trained person advises. Salt or bath treatments can help with parasites. Medicines need a withdrawal period, the waiting time before the fish may be sold, so no medicine stays in the fish that people eat. Overuse of antibiotics also makes germs resistant.

Try it: break the triangle

In the 3D pond, a germ is in some fish. Move the fish slider up and the water slider down and see how fast it spreads. Then do the opposite. Which side of the triangle did you remove?

Key formulas and definitions

Worked examples

1. In a tank of 200 fish, 30 are sick. What is the prevalence of the disease?

Prevalence = 30 / 200 × 100 = 15 percent.

2. A farmer stocks 10 000 young fish and 8 500 are alive at harvest. Find the survival rate.

Survival = 8500 / 10 000 × 100 = 85 percent. Mortality is 15 percent.

3. A bath needs 20 mg of a treatment chemical per litre in a tank with 500 litres. How many grams are needed?

20 mg × 500 = 10 000 mg = 10 g.

4. A farm stocks 4 fish per square metre in one pond and 12 per square metre in another. In which is a disease likely to spread faster, and why?

In the 12 per square metre pond. Fish touch each other more, water gets dirtier and fish are more stressed, so the germ jumps faster.

5. Why must a farmer wait before selling fish that were given medicine?

The medicine needs time to leave the fish body. This waiting time is the withdrawal period, and it keeps the food safe for people.

Common mistakes

Practice quiz

1. Which is a non-infectious cause of fish deaths?
2. Keeping new fish apart for some days before adding them to the pond is called:
3. If 24 of 300 fish are sick, the prevalence is:
4. The waiting time after medicine before fish may be sold is the:
5. Which action makes disease spread faster?

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

How do fish diseases spread in a pond?

Through water, touching, wild fish, birds, shared nets and boots, infected young fish and dirty feed. Crowding and poor water make it faster.

How can fish disease be prevented?

Keep water clean, avoid crowding, buy healthy young fish, quarantine new fish, wash equipment, feed well and use vaccines where they exist.

What is biosecurity in aquaculture?

It is a set of steps to keep germs out of a farm and stop them leaving: clean water, disinfected tools, quarantine and controlled visitors.

Where this is taught

Japan高校（専門学科）1〜3年Resource Enhancement (Aquaculture)

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